Since March, 1992 we have been studying gene targeting which is a recombinational event that produces exchange between DNA introduced into a cell and its homologous chromosomal target. Homologous recombination involves two major steps: pairing of DNA strands at the region of homology, and DNA strand exchange. Homologous recombination allows precise targeted insertion of genetic information, gene expression in appropriate tissues at appropriate levels, correction of dominant mutations, and gene inactivation. The goal of this research is to increase the frequency of homologous recombination events. It is known from experiments in bacteria and yeast that these events are mediated by proteins described as general recombinases. General recombination proteins function by recognizing and pairing DNA sequences on the basis of shared homology. The best characterized general recombinase is the bacterial recA protein. It plays a central role in homologous DNA recombination and repair. RecA alone catalyzes the pairing and strand exchange steps during homologous recombination. Previous work demonstrates that recA can perform its function on mammalian dna in vitro. This work is known as Achilles' cleavage or recA-assisted restriction endonuclease cleavage. RecA has also been expressed in yeast. A specific goal is to express recA in a mammalian system and study the effect upon homologous recombination. In addition, a recA producing cell line can be used for experiments involving triple helix formation. It is thought that triplex formation can be used in place of antisense technology.